Magnetic circuit and construction thereof



Jan. 28, 1936. B GE 2,029,282

MAGNETIC CIRCUIT AND CONSTRUCTION THEREOF Filed April 13, 1955 Patented Jan. 28, 1936 UNITED STATES PATENT OFFICE MAGNETIC CIRCUIT AND CONSTRUCTION THEREOF 7 Claims.

This invention relates to transducers and more particularly to electrical apparatus involving a magnetic circuit in which it is desired to produce strong magnetic fields within a relatively small air gap. While the invention is generally useful in all classes of electrical apparatus operating under such conditions, it is of particular utility in connection with so called dynamic loud speakers for the reproduction of sound as in the reception of radio programs and the reproduction of music from electrical phonographs and the like.

It will also be understood, however, that the apparatus is useful in connection with microphones, relays, and the like, in which a coil or armature is to be moved, either mechanically or by electrical forces while exposed to a relatively intense magnetic field.

Among the objects of my invention are the following: 1

To provide apparatus of the class described for producing in a gap in a magnetic circuit a relatively intense and constant magnetic field by the utilization of permanent magnets only, eliminating the necessity for exciting the magnetic circuit by means of a coil energized by direct current.

To provide apparatus of the class described in which the magnet material is utilized with the greatest efiiciency.

To provide apparatus of the class described in which a relatively strong magnetic field is produced with a minimum weight of magnet ma terial.

To provide apparatus of the class described in which the efficiency is maintained at a high value by the elimination of leakage.

To provide apparatus of the class described which is relatively simple and economical to manufacture involving relatively low cost for labor and material and in which the apparatus may be easily and quickly assembled by unskilled labor.

To provide apparatus of the class described requiring a minimum amount of machining in order to assemble the parts.

To provide a dynamic speaker energized by a permanent magnet, at a relatively low cost as compared with the present cost of such devices of equal efficiency.

Still other objects and advantages of my invention will be apparent from the specification.

The features of novelty which I believe to be characteristic of my invention are set forth with 55 particularity in the appended claims. My invention itself, however, both as to its fundamental principles and as to its particular embodiments, will best be understood by reference to the specification and accompanying drawing, in which Fig. l is a cross sectional view of permanent 5 magnet dynamic loud speaker constructed in accordance with my invention;

Fig. 2 is a perspective view of the magnet employed in Fig. l, and

Figs. 3 to 9, inclusive, are sectional views of 10 various modified forms of magnetic circuit.

In the various figures like reference numerals represent like parts.

Referring now more particularly to Fig. 1, l represents the cone housing, usually a conical 15 metal stamping secured to the outer pole plate 2, provided with a central, and preferably circular opening to which there is secured the yoke 3,

-which may carry the bar or short section of magnet material 8, and which may be provided 20 at its outer end with a cap 1 forming the inner pole piece, somewhat reduced in diameter at its outer end, the inner and outer pole pieces having suflicient clearance, preferably of the order of approximately 0.040" to define a relatively small annular air gap. The magnet 6 and the inner pole piece 1 may be secured in position in any suitable manner such as by means of bolt 8 passing through a central opening in the magnet and engaging a threaded opening in the pole piece. 30

A spacing ring 2| may also be provided, formed of non-magnetic material such as brass, for accurately positioning the inner pole piece. The spacing ring may surround the inner pole piece, and be secured to the outer pole plate in any suitable manner, as by rivets or bolts, not shown.

Suspended in the air gap, there may be provided a voice coil 9 wound upon the form 9, which in turn is secured to the diaphragm proper I I which may satisfactorily be a cone of light and stiff material, such as paper, having its rim in the form of a fiange clamped by clamping ring l2 into the cone housing and the flange may be provided with concentric corrugations to increase its flexibility.

For the purpose of centering the voice coil in the gap and of permitting it to vibrate, there may be provided a suspension 20 which may be in the form of a circular disk of paper or like material surrounding the voice coil form and secured thereto and clamp-ed at its periphery to a bracket secured to the outer pole plate. For the purpose of matching the impedance of the voice coil to an external source of supply of sound currents, there may be provided a transformer 05 l5 having windings H5. The leads from the primary winding indicated at H are to be connected to any suitable source of sound power, while the leads l8 from the secondary may be secured to the cone and thence carried to the terminal of the voice coil.

Assuming that the magnet 6 maintains within the air gap a unidirectional and constant magnetic field when sound currents are supplied to the voice coil, the voice coil, and consequently the diaphragm, is vibrated in accordance with such currents, setting up sound waves and, conversely, when the diaphragm is vibrated by sound waves, sound voltages corresponding thereto are induced in the voice coil, so that the apparatus is reversible and may be employed either as loud speaker or microphone.

The constructional features of such devices other than as hereinfater claimed do not form, per se, a part of this invention, and the diaphragm and voice coil construction is generally similar to that heretofore employed in dynamic speakers. In the past, however, it has not been possible to produce a permanent magnet dynamic speaker of light weight and small cost because magnet materials heretofore employed were capable of producing only relatively weak magnetic fields per unit of weight of magnet material, and this required the use of excessive amounts of metal in the magnetic circuit which, in turn, increased the cost of the apparatus, both initially in material, labor cost, and the transportation cost for shipping it.

I have discovered that these difficulties may be overcome by the use of special magnet material, preferably, magnetic alloys made up generally of iron, with the addition of nickel and aluminum and small amounts of cobalt, manganese, tungsten, molybdenum, vanadium, copper and/or carbon, with the magnetic circuit arranged as pointed out-herein. For example, I may utilize an alloy consisting of 5 to 40% nickel, 1 to 20% of aluminum and the balance iron, or from 5 to 30% of nickel, i to 85% aluminum, 1 to 5% chromium, and. up to 1% carbon, or an alloy containing from 5 to 30% of nickel, from 1 to 15% aluminum, from 1 to 5% chromium and from to 40% cobalt, the various proportions and amounts of the particular constituents being selected as desired for particular conditions.

Such alloys are generally characterized by high coercive force (He) and high residual magnetism (Br) and relatively small amounts of such materials produce relatively strong magnetic fields. Furthermore, such alloys are usually of low permeability.

A disadvantage of such alloys is that they may, in some cases, be extremely hard and can be machined only with difliculty, if at all, although they can be ground. Hence I prefer to provide the magnet in a shape which requires as little machining as possible, for instance, in the form shown in Fig. 2, in which the magnet may be relatively simple and economically cast in the form of a long bar and out to size by means of a cutting wheel, the cut faces being subsequently ground and polished, if necessary, butno machining of the cylindrical surface being required.

In-the form shown the center of the magnet may be cored out in the casting to provide an opening for the bolt 8, and the center pole piece may be exactly located and maintained in position by means of the spacing ring 2|, and the entire assembly may further be secured together by means of bolts 4 and 5, clamping the outer pole piece to the yoke.

An alternative form for constructional purposes may be generally similar to that shown in Fig. 6, in which there is provided a tapered shell 35 holding the outer pole piece 1 against the magnet 6 and in turn holding the magnet 6 against the yoke 3. Under such conditions the bolt 8 may be omitted and the central hole extending through the magnet may likewise be omitted, the casting being made in the form of a solid bar.

In Fig. 3 the magnet 30 has been utilized as the center pole piece and is supported by the sleeve or spacing ring 2!. This construction may be advantageously employed where the alloy utilized is capable of being machined without too great difiiculty.

The stem may be given various shapes for particular purposes if desired, as indicated in Fig. 4, in which 35 represents the bar magnet and spacing ring 25 is omitted, the yoke and outer pole piece being secured by clamps 40. The entire stern, instead of a portion of it, may be formed by the bar magnet as indicated by bar magnet 32 in Fig. 5.

In Fig. 6 there may be provided a central core magnet 3 surrounded by annular bar magnets 33, the whole assembly, including the inner pole piece 5, being held in position by sleeve 35 and spacing ring 2i.

Alternatively, the entire center pole piece may be made of soft iron and the magnet displaced from the air gap laterally, as indicated in Fig, 7, in which 36 represents the bar magnet.

Under certain conditions, for the purpose of securing a more uniform product and for other reasons, it may be desirable to provide the mag net 3? with annular grooves parallel to its axis, as indicated in Fig. 8, and under certain conditions the inner and outer faces of the frame may be made in one piece, as indicated in Fig. 9, the magnet being designated by 38 and the center pole piece by i. In every case the term magnet herein refers to alloys such as described above, and the remaining parts of the magnetic circuit which are subjected to magnetic flux are made of magnetic material such as soft iron or mild steel, or other suitable magnetic material having relatively high permeability at high flux density.

It will be noted that in the forms shown herein, the surfaces of the magnet which are joined to the remainder of the magnetic circuit are prefobtaining the highest possible flux density in the air gap per unit of magnet weight.

The central stem or pole piece is preferably constructed of iron or mild, steel forming a magnetic flux conductor having a homogeneous structure throughout its length. Furthermore, the pole piece has an under plane surface'contacting with the polar plane surface of the magnet, said plane surface having an area substantially the same as the area of the polar surface of the magnet (see Fig. 1), while the upper or outer end of said pole piece is preferably reduced in diameter. The polar plane surfaces of the magnet extend substantially normal to the axis of the magnet and normal to the flow of magnetic flux. The magnet as herein illustrated has a diameter greator than its length.

In the present transducer structure the magnet and pole piece cooperate to provide an assembly in which the flux lines are preferably concentrated in the iron or mild steel pole piece. The pole piece has different flux concentrations. The under plane surface of the pole piece, lying in contact with the polar plane surface of the magnet, is of suflicient area to maintain a relatively low flux density at this point in the magnetic circuit, whereby to reduce reluctance at the plane of contact between said magnet and pole piece.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of my invention, as will be clear to those skilled in the art.

I claim:

1. In a magnetic circuit for a transducer, a single relative short permanent magnet having a polar plane surface extending in a plane normal to the axis of the magnet and to the flux lines, a mild steel yoke attached to said magnet and including a mild steel pole plate having an opening therein concentric with the axis of said permanent magnet, a pole piece having a plane under surface extending normal to the axis of the pole piece and contacting with the polar plane surface of the magnet. said pole piece projecting into said opening and cooperating therewith to define an air gap between the pole piece and pole plate, said pole piece having a substantially homogeneous structure throughout its length but different flux concentrations.

2. In a magnetic circuit for a transducena single relative short permanent magnet having a polar plane surface extending in a plane normal to the axis of the magnet and to the flux lines, a mild steel yoke attached to said magnet and including a mild steel pole plate having an opening therein concentric with the axis of said permanent magnet, a pole piece having a plane under surface extending normal to the axis of the pole piece and contacting-with the polar plane surface of the magnet, said pole piece projecting into said opening and cooperating therewith to define an air gap between the pole piece and pole plate, said pole piece having a substantially homogeneous structure throughout its length but different flux concentrations, said plane under surface of said pole piece having an area greater than the cross sectional area of the pole piece portion projecting into said opening and of substantially the same magnitude as the area of the polar plane surface of the magnet, whereby to maintain substantially a relatively low flux density at the plane of contact between said magnet and pole piece as compared to the flux density at the air gap and to also minimize the reluctance at said plane of contact.

3. In a magnetic circuit for a transducer, a single relative short permanent magnet having a polar plane surface extending in a plane normal to the axis of the magnet and. to the flux lines, a mild steel yoke attached to said magnet and including a mild steel pole plate having an opening therein concentric with the axis of said permanent magnet, a pole piece having a plane under surface extending normal to the axis of the pole piece and contacting with the polar plane surface of the magnet, said pole piece projecting into said opening and cooperating therewith to define an air gap between the pole piece and pole plate, said pole piece having a substantially homogeneous structure throughout its length but different flux concentrations, said magnet being constructed of substantially uniform cross-section throughout its length whereby to eliminate concentration of flux lines in said magnet, said pole piece being constructed and arranged to concentrate said flux lines subsequent to the flow of same from said magnet to said pole pieces.

4. In a magnetic circuit for a transducer, a single relatively short cylindrical permanent magnet centrally positioned in said circuit and having a polar plane surface extending in a plane normal to the axis of the magnet and to the flux lines, said circuit including a return circuit comprising a mild steel yoke and pole plate, said pole plate having a central opening, a pole piece separate from said magnet and constructed of mild steel having a substantially homogeneous structure throughout its length, said pole piece attached to the polar plane surface of the magnet and axially aligned with said magnet and projecting into said opening, said pole piece and pole plate cooperating to provide an annular air gap having an axis extending coaxially with respect to said magnet and pole piece axes, said magnet having a diameter greater than its length.

5. In a magnetic circuit for a transducer including a yoke, a pole plate, and a stem portion, said yoke and pole plate constructed of mild steel, said pole plate having an opening cooperating with said stem to provide an annular air gap, said stem constructed of spaced stem portions of mild steel and an intermediate permanent magnet portion.

6. In a magnetic circuit for a transducer including a yoke, a pole plate, and a stem portion, said yoke and pole plate constructed of mild steel, said pole plate having an opening cooperating with said stem to provide an annular air gap, said stem constructed of spaced stem portions of mild steel and an intermediate permanent magnet portion, said magnet having opposite polar plane surfaces respectively contacting with complementary plane surfaces carried by said steel stem portions, and extending normal to the magnet axis and to said flux lines, the axial length of said magnet being less than the mean width of said polar plane surfaces of said magnet.

'7. In a magnetic circuit for a transducer, including a yoke, a pole plate, and a stem portion, said yoke and pole plate constructed of mild steel, said pole plate having an opening cooperating with said stem to provide an annular air gap, said stem constructed of spaced stem portions of mild steel and an intermediate permanent magnet portion, said magnet having opposite polar plane surfaces respectively contacting with complementary plane surfaces carried by said steel stem portions and extending nor-. 

